Present day automobile airbag safety systems rely on sensors to provide indication during a crash that the crash is of sufficient severity to warrant the deployment of an airbag. These sensors are reactive in the sense that they can only measure the response of the car during the actual physical crash. The sensor system however, must provide adequate warning to permit airbag deployment. A general rule of performance is the "5 inch--30 millisecond" rule: as a general norm, an airbag must be fully deployed after a travel of 5 inches in the front seat of the passenger compartment, where the travel is defined as the integration of the velocity change during the accident at the location of the passenger compartment. Since it takes approximately 30 milliseconds to deploy a passenger side airbag fully, the sensor system must provide an indication 30 milliseconds before the front seat has traveled 5 inches during the crash. In assessing this requirement it is helpful to recall that a vehicle traveling at 60 miles per hour is traveling at 88 feet per second or 0.88 feet per 10 milliseconds.
In addition to providing this advance indication, the sensor system must be capable of separating "must-fire" crashes from "no-fire" crashes, since not all crashes are severe enough to warrant the deployment of an airbag. Typically for a frontal crash, a velocity of 14 mph separates crashes requiring an airbag from those that do not require an airbag.
Sensors that are employed include mechanical, electromechanical and electronic devices. A mechanical sensor might involve the movement of a mass against a restraint arm. If the movement is sufficient, a spring loaded firing pin is released, puncturing a primer that initiates the airbag firing. In an electromechanical sensor, such as the ball-in-tube sensor, an electrical contact is closed if a magnetically restrained ball breaks free and closes the contacts of an external circuit. Both mechanical and electromechanical sensors are located near the point of initial contact, i.e. the front of the vehicle for frontal crashes. More than one sensor is usually required. In the mechanical and electromechanical sensor systems, the separation of "must-fire" crashes from "no-fire" crashes is accomplished with bias and damping parameters built into the sensor design, since these sensors are basically switches.
Electronic sensors rely on micromachined silicon capacitive or piezoresistive accelerometers. These sensors are typically located on a structural component close to the front of the passenger compartment, and measure the acceleration along the longitudinal axis of the car. The output of the electronic sensor is a voltage proportional to the acceleration along the axis of the vehicle. A microcontroller continually monitors the electronic sensor output and by means of a suitable algorithm determines if a crash is occurring and if it is severe enough to warrant airbag deployment.
Whereas mechanical and electromechanical systems typically require several sensors, some of which are located close to the front of the vehicle, and a system diagnostic unit, the electronic sensor can be configured as a single unit. Because of the advantages of this arrangement, the present industry trend is towards a single electronic sensor located on a structural component near the front of the passenger compartment.
The response of any sensing system is both vehicle specific and crash specific. While some vehicles and some crashes are relatively easy for the sensing system to diagnose, others are not. Two types of condition present particular difficulty: pole crashes and rough road conditions. In the case of pole crashes, it has been found that a pole can effectively slice through the front of the vehicle a considerable distance, using up valuable time, until the signature of a severe crash is recognized. In the latter case, rough roads can provide false indications of a crash.
For side impact crashes, the situation is more severe since the extent of the vehicle between the impacting object and the vehicle interior is much less than frontal crashes, providing less time for interpretation of data and for an airbag deployment decision.
In addition to these considerations, potentially adverse consequences of full airbag deployment when passengers are out of position, are leading to the development of "smart" airbag systems that deploy on the basis of occupant size and position. Pretensioning of seat belt restraints and integration of seat belt systems with "smart" airbag systems is also under development.